The invention relates to a method for the production of a printing form by the scanning point-by-point and line-by-line of an image pattern and engraving cavities along corresponding cylindrical peripheral engraving lines with the use of a screen.
Generally, an image pattern which is to be reproduced is mounted on a scanning or image cylinder which is rotated during the process. The image pattern is scanned helically by a scanning device such as an optical scanner which is continuously guided parallel to the axis of the image cylinder. The image pattern can contain half-tone images and/or characters. The tone value of the scanned image points determine the amount of the reflected light and the reflected light is detected and converted into image signals. In order to produce a photogravure screen on the engraving cylinder, a screen signal is superimposed over the image signals.
The engraving device includes an engraving member such as a mechanical engraver which uses an engraving needle as a cutting tool. The engraving member moves continuously along the rotating engraving cylinder.
The screen signal creates a vibrating lifting movement of the engraving needle and the image signals determine the penetration depth of the engraving needle or cutting tool into the surface of the engraving cylinder. Thus, a series of cavities of varying depths are produced along a helical line at the surface of the engraving cylinder.
In one prior art system, the image and engraving cylinders are each driven by a separate synchronous motor. Another known system uses a single synchronous motor to rotate the image and engraving cylinders which are rigidly connected axially to each other.
The engraving cylinder is used in a photogravure rotary machine for a printing process. The cylinder is inked and the cavities retain an amount of ink depending upon the respective cavity depth. During the printing process, ink is transferred from the cavities to a printing carrier and the tone-value of the printed surface is determined by the depth of the respective cavities which are disposed in a screen defined on the cylinder surface. For multi-color printing, cavities for each color are engraved on a single cylinder. In the photogravure rotary machine, a color print is produced by superimposing the printing from different engraving cylinders for the respective colors.
In the case where a screen is used, a screen signal produces a vibrating stroke motion for the engraving needle so that a series of cavities are engraved in the rotating engraving cylinder. The image signal determines the depth of the cavities with the black portion of the image pattern corresponding to the greatest depth whereas the white portion of the image pattern corresponds to shallow cavities.
In a method for the production of a printing form by the scanning point-by-point and line-by-line of an image pattern along successive cylindrical peripheral image lines and engraving cavities along corresponding cylindrical peripheral engraving lines, one known process interrupts the scanning during the time the engraving member is moved from one peripheral engraving line to the next. In addition, the engraving needle is abruptly moved into a rest position before engraving the next line and in this position the engraving needle is no longer in contact with the surface of the engraving cylinder. The signal for engraving the first cavity in a line causes an abrupt change in the position of the engraving needle from the rest position to the engraving depth corresponding to the respective image signal.
Generally, between the tone-value for a black portion of the image pattern and the white portion of the image pattern there can be 200 tone-value steps or increments.
The stroke difference for the engraving needle for such a tone-value range amounts to about 0.15 u/m. Irregularities in the tone-value engraved can be disturbing to a person viewing the image printed therefrom.
In order to attain a good reproduction quality, the engraving of the cavities must be done with great precision and this imposes stringent requirements on the mechanical system used in the engraving step.
Generally, an engraving member comprises a stationary electromagnet and a movable rotatable system. The rotatable system includes a shaft connected to a fixed portion, a bearing for the shaft, and a dampening arrangement.
One end of the shaft is connected to a clamped torsion rod and the other end of the shaft carries an arrangement including the engraving needle. The current in the electromagnet is proportional to the image signals and the screen signals and causes a magnetic field which exerts a torque on the torsion rod and thereby results in an angular deflection of the engraving needle from its rest position established by the torsion rod.
The change in the engraving needle from its rest position to its engraving position is abrupt and is dampened by the dampening arrangement in order to prevent overshoot and to minimize uncontrolled oscillations.
Despite the dampening arrangement, it has been found that the sudden change of the engraving needle from its rest position to the engraving of the first cavity in a peripheral line results in a cavity deeper or smaller than it should be according to the image signal. In addition, uncontrolled oscillations of the engraving needle cause undesired cavities to be engraved.
These problems arise because the change of the engraving needle from its rest position to its engraving position and vice versa is a comparatively large stroke and the momentum cannot be dissipated effectively without compromising other aspects of the engraving process. The defects from these problems reduce the reproduction quality considerably.
The instant invention endeavors to eliminate the problems usually associated with the changes of the engraving needle from its rest position to the engraving position and vice versa.